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Context. The study of gas-rich debris discs is fundamental to characterising the transition between protoplanetary discs and debris discs. Aims. We determine the physical parameters of the brightest gas-rich debris disc orbiting HD 141569A. Methods. We analyse images from the NOrthern Extended Millimeter Array (NOEMA)1 and the Atacama Large Millimeter/submillimeter Array (ALMA) in 12CO, 13CO J = 2→1, and 13CO J = 1→0 transitions. We incorporate ALMA archival data of the 12CO J = 3→2 transition and present continuum maps at 0.87, 1.3, and 2.8 mm. We use simple parametric laws with the Diskfit code and MCMC exploration to characterise the gas disc parameters and report a first attempt to characterise its chemical content with IRAM-30 m. Results. The continuum emission is equally shared between a compact (≤50 au) and a smooth, extended dust component (~350 au). Large millimetre grains seem to dominate the inner regions, while the dust spectral index is marginally larger in the outer region. The 12CO is optically thick, while 13CO is optically thin with 13CO ~ 0.15 (C18O is not detected). The 13CO surface density is constrained to be one order of magnitude smaller than around younger Herbig Ae stars, and we derive a gas mass M12CO = 10-1M®. We confirm the presence of a small CO cavity (RCO = 17 × 3 au), and find a possibly larger radius for the optically thin 13CO J = 2→1 transition (35 × 5 au). We show that the observed CO brightness asymmetry is coincident with the complex ring structures discovered with VLT/SPHERE in the inner 90 au. The 12CO temperature T0(100 au) ~ 30K is lower than expected for a Herbig A0 star, and could be indicative of subthermal excitation. Conclusions. With the largest amount of dust and gas among hybrid discs, HD 141569A shows coincident characteristics of both protoplanetary discs (central regions), and debris discs at large distance. Together with its morphological characteristics and young age, it appears to be a good candidate to witness the transient phase of gas dissipation, with an apparently large gas-to-dust ratio (G=D > 100) favouring a faster evolution of dust grains. [ABSTRACT FROM AUTHOR]

Context. ALMA high angular resolution observations of the dust and CO emission have already revealed signatures of protoplanets embedded in protoplanetary disks. These detections are around single T Tauri stars, while exoplanet surveys reveal that planets can also form in binary (or multiple) systems, either in circumstellar or circumbinary orbits. Aims. We searched for indirect evidence for planet formation in the multiple system GG Tau A, which harbors the most massive circumbinary disk among T Tauri stars. Methods. We performed CO(2-1) ALMA Cycle 6 observations of GG Tau A at 0.300 resolution. The images confirm the "hot spot" detected at higher frequencies, but also reveal prominent spiral-like features. We modeled these features using the analytic prescription for the linear perturbation regime induced by low-mass planets. Results. The brightest spiral is well reproduced by a density wave excited by a protoplanet (GG Tau Ac) at the hot-spot location (290 au), just outside the dust ring. The absence of a clear gap (in gas or dust) at the planet location implies that its mass is significantly lower than that of Jupiter, i.e., of about the mass of Neptune or lower. Furthermore, other prominent (trailing) spiral patterns can be represented by adding one (or more) planet(s) at larger orbital radii, with the most obvious candidate located near the 2:1 mean-motion resonance with GG Tau Ac. Conclusions. The (proto-)planet GG Tau Ac appears to externally confine the ring in a stable configuration, explaining its high mass. Our results also suggest that planets similar in mass to Neptune may form in dense circumbinary disks orbiting (wide) binary stars. In the GG Tau case, orbital resonances appear to play an important role in shaping this multiple circumbinary planet system. [ABSTRACT FROM AUTHOR]

Context. Determining the gas density and temperature structures of protoplanetary disks is a fundamental task in order to constrain planet formation theories. This is a challenging procedure and most determinations are based on model-dependent assumptions. Aims. We attempt a direct determination of the radial and vertical temperature structure of the Flying Saucer disk, thanks to its favorable inclination of 90 degrees. Methods. We present a method based on the tomographic study of an edge-on disk. Using ALMA, we observe at 0.500 resolution the Flying Saucer in CO J = 2–1 and CS J = 5–4. This edge-on disk appears in silhouette against the CO J = 2–1 emission from background molecular clouds in ρ Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity provide a direct measure of the gas temperature as a function of radius and height above the disk mid-plane. Results. The overall thermal structure is consistent with model predictions, with a cold (<12−15 K) CO-depleted mid-plane and a warmer disk atmosphere. However, we find evidence for CO gas along the mid-plane beyond a radius of about 200 au, coincident with a change of grain properties. Such behavior is expected in the case of efficient rise of UV penetration re-heating the disk and thus allowing CO thermal desorption or favoring direct CO photo-desorption. CO is also detected at up to 3–4 scale heights, while CS is confined to around 1 scale height above the mid-plane. The limits of the method due to finite spatial and spectral resolutions are also discussed. Conclusions. This method appears to be a very promising way to determine the gas structure of planet-forming disks, provided that the molecular data have an angular resolution which is high enough, on the order of 0.3−0.1 00 at the distance of the nearest star-forming regions [ABSTRACT FROM AUTHOR]

Context. The increased sensitivity of millimeter-wave facilities now makes possible the detection of low amounts of gas in debris disks. Some of the gas-rich debris disks harbor peculiar properties, with possible pristine gas and secondary generated dust. The origin of the gas in these hybrid disks is strongly debated and the current sample is too sparse to understand this phenomenon. Aims. More detections are necessary to increase the statistics on this population. Lying at the final stages of evolution of protoplanetary disks and at the beginning of the debris disk phase, these objects could provide new insight into the processes involved in the making of planetary systems. Methods. We carried out a deep survey of the CO J = 2 → 1 and CO J = 3 → 2 lines with the APEX and IRAM radiotelescopes in young debris disks selected according to hybrid disk properties. The survey is complemented with a bibliographic study of the ratio between the emission of the gas and the continuum (SCO/Fcont) in CTTS, Herbig Ae, WTTS, hybrid, and debris disks. Results. Our sub-mm survey comprises 25 stars, including 17 new targets, and we increase the sensitivity limit by a factor 2 on eight sources compared to similar published studies. We report a 4σ tentative detection of a double-peaked CO J = 2 → 1 line around HD 23642; an eclipsing binary located in the Pleiades. We also reveal a correlation between the emission of the CO gas and the dust continuum from CTTS, Herbig Ae and few debris disks. The observed trend of the gas to dust flux ratio suggests a concurrent dissipation of the dust and gas components. Hybrid disks systematically lie above this trend, suggesting that these systems may witness a transient phase, when the dust has evolved more rapidly than the gas, with a flux ratio SCO/Fcont enhanced by a factor of between 10 and 100 compared to standard (proto-)planetary disks. [ABSTRACT FROM AUTHOR]

Aims. We attempt to determine the molecular composition of disks around young low-mass stars. Methods. We used the IRAM 30 m radio telescope to perform a sensitive wideband survey of 30 stars in the Taurus Auriga region known to be surrounded by gaseous circumstellar disks. We simultaneously observed HCO+ (3−2), HCN(3−2), C2H(3−2), CS(5−4), and two transitions of SO. We combined the results with a previous survey that observed 13CO (2−1), CN(2−1), two o-H2CO lines, and another transition of SO. We used available interferometric data to derive excitation temperatures of CN and C2H in several sources. We determined characteristic sizes of the gas disks and column densities of all molecules using a parametric power-law disk model. Our study is mostly sensitive to molecules at 200−400 au from the stars. We compared the derived column densities to the predictions of an extensive gas-grain chemical disk model under conditions representative of T Tauri disks. Results. This survey provides 20 new detections of HCO+ in disks, 18 in HCN, 11 in C2H, 8 in CS, and 4 in SO. HCO+ is detected in almost all sources and its J = 3−2 line is essentially optically thick, providing good estimates of the disk radii. The other transitions are (at least partially) optically thin. Large variations of the column density ratios are observed, but do not correlate with any specific property of the star or disk. Disks around Herbig Ae stars appear less rich in molecules than those around T Tauri stars, although the sample remains small. SO is only found in the (presumably younger) embedded objects, perhaps reflecting an evolution of the S chemistry due to increasing depletion with time. Overall, the molecular column densities, and in particular the CN/HCN and CN/C2H ratios, are well reproduced by gas-grain chemistry in cold disks. Conclusions. This study provides a comprehensive census of simple molecules in disks of radii >200−300 au. Extending that to smaller disks, or searching for less abundant or more complex molecules requires a much more sensitive facility, i.e., NOEMA and ALMA. [ABSTRACT FROM AUTHOR]

Context. Dust determines the temperature structure of protoplanetary disks, however, dust temperature determinations almost invariably rely on a complex modeling of the Spectral Energy Distribution. Aims. We attempt a direct determination of the temperature of large grains emitting at mm wavelengths. Methods. We observe the edge-on dust disk of the Flying Saucer, which appears in silhouette against the CO J = 2-1 emission from a background molecular cloud in Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity allows us to directly measure the dust temperature. The dust opacity can then be derived from the emitted continuum radiation. Results. The dust disk absorbs the radiation from the CO clouds at several velocities. We derive very low dust temperatures, 5 to 7 K at radii around 100 au, which is much lower than most model predictions. The dust optical depth is >0.2 at 230 GHz, and the scale height at 100 au is at least 8 au (best fit 13 au). However, the dust disk is very flat (flaring index -0.35), which is indicative of dust settling in the outer parts. [ABSTRACT FROM AUTHOR]

Aims. We attempt to determine the molecular composition of disks around young low-mass stars in the Oph region and to compare our results with a similar study performed in the Taurus-Auriga region. Methods. We used the IRAM 30 m telescope to perform a sensitive search for CN N = 2-1 in 29 T Tauri stars located in the ρ Oph and upper Scorpius regions. 13CO J = 2-1 is observed simultaneously to provide an indication of the level of confusion with the surrounding molecular cloud. The bandpass also contains two transitions of ortho-H2CO, one of SO, and the C17O J = 2-1 line, which provides complementary information on the nature of the emission. Results. Contamination by molecular cloud in 13CO and even C17O is ubiquitous. The CN detection rate appears to be lower than for the Taurus region, with only four sources being detected (three are attributable to disks). H2CO emission is found more frequently, but appears in general to be due to the surrounding cloud. The weaker emission than in Taurus may suggest that the average disk size in the ρ Oph region is smaller than in the Taurus cloud. Chemical modeling shows that the somewhat higher expected disk temperatures in ρ Oph play a direct role in decreasing the CN abundance. Warmer dust temperatures contribute to convert CN into less volatile forms. Conclusions. In such a young region, CN is no longer a simple, sensitive tracer of disks, and observations with other tracers and at high enough resolution with ALMA are required to probe the gas disk population. [ABSTRACT FROM AUTHOR]